Articular cartilage is a highly specialized tissue that covers the surfaces of long bones to allow almost frictionless motion under large loads. In the healthy skeleton, this articulating function allows bones to change their relative angular relationship about a joint, as in the hip and the knee joints. This function of joints occurs painlessly and virtually without additional effort due to the low friction of mating joint surfaces which arises from the properties of the synovial fluid within the joint, and the smooth topography of the cartilage surfaces.
Although cartilage contains its own specialized cells (chondrocytes), it has limited ability to repair injuries, which may range from localized tears, to focal areas of loss of coverage of the underlying bone, to degenerative conditions, such as osteo- and rheumatoid arthritis in which the entire cartilage layer and underlying (subchondral) bone can be affected. Generalized or degenerative conditions, most commonly osteoarthritis, are frequently treated with total joint replacement in which the cartilage surface and underlying bone are completely replaced with artificial materials that articulate with minimal friction. In cases where the area of cartilage loss or mechanical damage is limited to a focal location within otherwise normal cartilage, the treatment of choice is to replace the damaged tissue with a graft taken from some other site within the patient's joint (e.g. the trochlea of the distal femur) or from an anatomically similar location in a bone obtained from, e.g., an animal donor (xenograft) or more preferably from a human donor. Grafts harvested from the patient's own body are termed “autografts”, while grafts harvested from human cadavers are termed “allografts”. Cartilage grafts are typically harvested in the form of a cylindrical core consisting of a surface layer of cartilage attached to a layer of subchondral bone which is in continuity with the spongy (cancellous) bone deep to the joint surface. These composite (osteo-chondral) grafts are machined (drilled) from sites within the joint (the “donor site”), and are designed to accurately match the dimensions of holes machined (drilled or cored) into the patient's joint at the site of cartilage damage (the “recipient site”).
The sites available for harvesting of cartilage grafts within the body are limited and present a risk of secondary oeteoarthritis due to the loss of the weight-bearing area of the native joint. In addition, as these donor sites are typically located within the distal femur at the periphery of the articular surface, the diameter of autografts harvested in this manner is typically limited to 10 mm. Because of these limitations, there is rapidly growing interest in the use of osteo-chondral grafts derived from cadaveric specimens, in which the cartilage cells have been kept alive for transplantation to the host recipient site.
Numerous published articles have documented favorable clinical outcomes after treatment of cartilage defects with osteochondral allografts, with few cases of immunogenic reactions or transmission of disease when the guidelines of the American Association of Tissue Bank have been observed. Despite these favorable results, practical obstacles impede the use of live allografts and their efficacy in the reconstruction of chondral surfaces. The Musculoskeletal Transplant Foundation has reported that nearly 13% of donor grafts surpass their expiration date and are disposed of before they can be utilized. This waste of donor tissue arises from two limiting factors: (i) the viability period of the graft, and (ii), difficulties in selecting a graft of the appropriate size and shape to match the anatomy of the recipient.
After harvesting a live articular specimen (e.g. a femoral condyle) and placing it in a bath of nutrient solution, the cell population within the cartilage gradually dies, causing deterioration of the load-bearing properties of the tissue. After approximately 4 weeks, the graft is degraded to such an extent that it becomes unusable for transplantation. It may be assumed that a period of two weeks is required to harvest, process, test, and catalogue the donated tissue. This means that the time available to then match the size and shape of the harvested bone to a patient and then to obtain the donor specimen and perform the implantation procedure is only a period of approximately two weeks.
A further consideration is the correspondence between the surface geometry (i.e. topography) of the graft that is implanted at surgery versus the surface geometry of the original chondral surface at the recipient site. This is of critical significance to the success of the grafting procedure. If the surface of the implanted plug protrudes above the original articular surface, the cartilage surface of the graft will be subject to excessive stress during weight-bearing and articulation and will not survive. This situation arises (i) when the curvature of the chondral layer of the graft is smaller than the original surface (i.e. the original cartilage is flatter than the plug), and (ii) when the hole machined in the host bone to receive the transplanted core is too shallow or is not correctly aligned with respect to the surrounding surface. Conversely, if the surface of the implanted graft is lower than that of the original articular surface, the cartilage immediately surrounding the graft will be subject to higher stresses, leading to damage during weight-bearing and articulation. This configuration is present when the surface of the graft is lower than the matching edge of the surrounding cartilage at some points at the graft periphery, or when the edge of the graft is aligned with the surface of the joint at the periphery but undergoes less elevation at the center of the graft compared to the original joint surface. In either situation enlargement of the area of cartilage degeneration may occur over a larger area than the original cartilage defect, and thus failure of the procedure to restore the health of the joint. Typically, this situation arises when the curvature of the chondral layer of the graft is larger than the original surface (i.e. the surface of the graft is flatter than the surrounding joint surface), or the hole drilled in the host bone to receive the transplanted core is too deep or is not correctly aligned with respect to the surrounding surface. Clinical experience has shown that a surface mismatch of less than approximately 0.5 mm is acceptable without adversely affecting the success of the procedure.
As each joint is unique in terms of the geometry of its articulating surfaces, any graft derived from another bone will have areas of imperfect topographic match, even if implanted perfectly. As the curvature of articular surfaces is approximately proportional to the gross dimensions (i.e. the overall width and depth) of the joint surface, the chances of obtaining an allograft that will yield a graft of acceptable curvature increase if the allograft is of approximately the same size as the patient's joint. However, this requirement significantly limits the number of allografts available to provide grafts for any individual patient with an articular defect that requires cartilage transplantation.
The current practice for selecting articular specimens to provide a cartilage graft for treatment of a specific recipient lesion simply involves matching the overall dimensions of the joints of donors and recipients, without regard to the topographies of the cartilage surfaces of the donor and recipient joints. For example, if a graft is requested for a patient with a defect of the medial femoral condyle (MFC) of the right knee, measurements will be taken of the length and width of the patient's right femoral condyle on knee radiographs. The tissue bank will then match these radiographic dimensions with the population of medial condylar allografts that it has on-hand using a tolerance of ±2 mm for the matching procedure. When this method of matching allografts to patients is performed, many requests for bony tissue go unfulfilled, increasing the wait-time for patients. Furthermore, the use of linear dimensions for donor/patient matching ignores the variation in curvature amongst condyles of a given size. This leads to significant challenges in achieving acceptable host-recipient matches during surgery and potential functional problems with graft incorporation and cartilage viability post-operatively. Therefore it is apparent that there exists a need to reduce waste, patient wait-time, and improve function through more efficient selection of donor graft sources using matching methods based on surface curvature of the donor and recipient joints.
It is the object of the current disclosure to provide a method and system in which donor articular cartilage surface curvatures are precisely matched to the patient's natural curvature while reducing wait-time and minimizing waste. This method and system will allow a surgeon to essentially order a graft of any given curvature to repair a cartilage defect and to receive said graft in a timely manner on a pre-determined date.